Significance & Research: Balanced chromosomal rearrangements represent both clinical diagnostic quandaries and exceptional experimental opportunities in human genetics as they offer a unique window into the impact of single locus hemizygosity in human disease. However, their contribution to complex disorders remains largely unquantified as they are not detected by conventional association approaches. Failure to consider BCRs bypasses a powerful complement to conventional association approaches in complex disease as they can directly implicate a causal locus or sequence motif, and may help explain a portion of the missing heritability in disorders such as autism spectrum disorders (ASDs). In this proposal, the candidate will delve into this unexplored genomic space by leveraging novel sequencing techniques innovated during his current NRSA to evaluate the full spectrum chromosomal aberrations that can impact human developmental abnormalities such as ASD, their inheritance, and the mechanism by which they arise. The proposed studies were carefully designed to develop expertise in three primary training domains; mechanism of DNA breakage repair and formation of chromosomal aberrations, clinical genetics and heterogeneous phenotypic presentation, and the molecular genetic consequences of chromosomal abnormalities on gene expression (transcriptomics). These skills are needed to establish expertise required to become a leader in the genomics of human neurodevelopmental abnormalities and chromosomal aberrations. Hypotheses: The aims of this proposal were designed to test the specific hypotheses supported by the preliminary data that: (1) inverted genomic segments represent an underappreciated and profound genetic risk factor mediating human chromosomal aberrations and complex chromosomal rearrangements by aberrant repair of small de novo or inherited local inversions (Aim 1), (2) phenotypic discordance from highly penetrant genetic lesions is mitigated by unrecognized genetic structure (Aim 2), and (3) balanced chromosomal aberrations underlie a meaningful portion of the unexplained genetic etiology of children with autism and no detectable dosage imbalance (Aim 3). Training: All research will be conducted within the Center for Human Genetic Research at MGH, Harvard Medical School, and the Broad Institute under the mentorship of James F. Gusella, Ph.D., an established leader in the field with a prolific record of discovery in human genetics. Training will be carried out in three primary domains with contributing experts in each field, including A) studying the mechanism of DNA break repair and chromosomal rearrangements with James Lupski, Ph.D., external advisory panel member, B) deep training in clinical genetics to understand the diverse phenotypes associated with neurodevelopmental abnormalities with Cynthia Morton, Ph.D., advisory panel member and Director of Cytogenetics at Harvard Medical School, and C) molecular genetics, transcriptomics, and the impact of chromosomal aberrations on gene expression with James Gusella, Ph.D. Director of the Center for Human Genetic Research and a leader in the molecular genetics of human disease and Mark J. Daly, Chief of the Analytical and Translational Genetics Unit of CHGR, expert in computational genomics, and emerging leader in autism genetics research. In addition to research training, the candidate will undertake coursework through Harvard University and MIT, participate in regular seminars and symposia, continue to lead an autism genomics group, and attend annual scientific meetings. Significance: The impact of balanced chromosomal aberrations in autism and other human developmental abnormalities is largely unknown as they remain completely undetectable by most genetic research designs. As the population prevalence of autism continues to increase, estimates at cytogenetic resolution suggest the impact of chromosomal abnormalities in these children is potentially high (estimated at an approximately six- fold increase in the development of autism). These studies will fulfill a vital need in the study of human developmental abnormalities and could provide significant insight into the mechanism by which these events occur and ultimately yield sequence specificity and predictive diagnostics to the patients studied in Aim 3. Overall, the training environment is exceptional, the proposed studies are innovative, the science is timely, the hypotheses address unresolved and important questions in the field that could yield seminal findings in autism genetics, the genomics of chromosomal organization, and the implementation of clinical diagnostics. The mentoring and research skills developed over the course of this award will undoubtedly provide a strong foundation for the candidate to become a successful independent scientist and leader in understanding the genomics underlying human developmental abnormalities. Indeed, the candidate's enthusiasm is very high for the remarkable training and research opportunities afforded in this application.